Systems for generating steam or so-called steam-generating boilers based on a reactor for so-called "quick-circulating" fluidization are known from commercial constructions. "Fast" fluidization occurs in a flow of combustion gases and air directed almost vertically upward, in which a granular material is carried and substantially entrained upward by the gas. This material consists of a fuel, e.g. coal and ash products from coal having, if necessary, an admixture of limestone for absorption of sulphur or an inert material such as sand. In most cases, the rate of flow is 3-8 m/s, and the size of the flowing grains is extremely small, i.e. in the micrometer range, up to some millimeter. The quantity of solid material may vary from low values at low load, up to twenty or more kg/m.sup.3 at high load.
Most of the entrained solid material is separated in a particle separator--for example a cyclone type separator--when flowing out from the top of the reactor and is "circulated" to the lower part of the reactor so as to:
a) maintain a suitable material density and sojourn time in the reactor,
b) obtain an excellent combustion reaction, and
c) obtain an excellent reaction of absorption for e.g. sulphur separation with an admixture of limestone.
Such a reactor is shown in FIG. 1.
The reactor is further characterized in that mainly by introduction of primary air into the bottom part and secondary air at a suitable level thereabove, a situation is, in practice, established in which a lower speed is obtained in the bottom part and a higher speed thereabove, which inter alia gives a higher density of solid material in the bottom part (in many cases from 100 to 600 kg/m.sup.3), where fuel can be degassed and partly burned. Large fuel particles and other solid materials stay or are enriched in this zone until they are burned out completely or disappear through a special material outlet in the bottom part. In operation, the reaction temperature is 750.degree.-1000.degree. C., however preferably 825.degree.-900.degree. C. in the combustion of coal.
To cool the system and recover the required part of the developed power of combustion, two techniques are used today. One implies that cooled surfaces, for example vertical tube surfaces cooled by water or steam, are arranged on the walls of the reactor or as internal baffles or the like disposed in the reactor. The other technique which is sometimes also combined with the first, implies that further power outputs are provided in that the flow of particles which is separated in said particle separator at the top of the reactor is wholly or partly conducted to an ash cooler of some suitable type before being reintroduced into the reactor. Of course, the power output is determined also by the amount of hot gas which is leaving the reactor.
Technically seen, there now is a situation where a first combustion reaction occurs in the bottom part of the reactor having the above-mentioned higher density of solid material, whereupon the final combustion of gases expelled from the fuel and burning-out of the coke particles formed occur higher up where the oxygen content has been increased by addition of secondary air.
For different reasons, it is not suitable to arrange heat-absorbing metallic surfaces in the bottom part of the reactor. One reason is the low oxygen partial pressure which easily causes corrosion on metallic surfaces and/or erosion.
The absorption of heat on cooling surfaces arranged on the reactor walls occurs through radiation from particles and gas supplemented with convective gas cooling towards the wall and more or less direct particle contact, whereby also large amounts of heat can be transferred. At full load, the heat transfer is typically between about 140.degree. and about 250 W/m.sup.2 .degree.C. depending on the temperature and the current particle load, when an optimal combustion of coal is desired.
In large reactors, it is constructionally difficult to arrange a sufficient cooling surface in the walls only, if the reactor is not made extremely high. Normally it is not considered economically optimal to make the reactor higher than required for a favourable combustion reaction. For this reason, the above-mentioned techniques of arranging cooling surfaces inside the reactor or of cooling the ashes before being reintroduced into the reactor, are used as a complement.
To obtain an optimal combustion reaction and absorption of sulphur, it must be possible to control a reactor of the above-mentioned type such that combustion of e.g. coal takes place in a relatively narrow range of temperature of about 850.degree.-875.degree. C. at full load and partial load. It has become apparent that natural parameters to be influenced are
1) the total contents of solid material in the reactor,
2) the part of this material, which is kept floating (i.e. the load of material) in the upper part with its cooling surfaces (which directly affects the coefficient of heat transfer),
3) the distribution of grains of the solid material (where a high amount of fine grains yields high coefficients of heat transfer),
4) recirculation of colder combustion gases (which increases the amount of heat carried away from the reactor by the gases) and
5) more or less cooling of the solid material which is separated after the reactor before this material is recycled.
Generally seen, it is known that such a reactor is to a certain extent self-adjusting, since if the flows of air to the bottom zone vary according to the load, the amount of material which is kept floating by the gas and, thus, the absorption of heat increase or decrease.
In constructional respect, the problems of obtaining an adequate position of the cooling surfaces increase according to the size of the reactor and the steam data (pressure and temperature) which the steam generator is to generate. Cooling surfaces such as tubes or bundles of tubes which are disposed inside the reactor are readily subjected to erosion under the action of the high flow of solid particles. Coolers for material separated in e.g. a cyclone are bulky, expensive and difficult to locate in large installations. In fact, they are units that require a very large space at the side of the reactor, and in addition to this, there are designing problems with ducts and the handling of high flows of material which are to be introduced into and discharged from the reactor.